1. Field of the Invention
The present invention relates generally to current telemetering apparatus wherein a receiver is connected to a remotely located transmitter by means of a two-wire transmission line or link. Specifically, the invention relates to interface apparatus which is commonly connected between such a receiver and its associated two-wire link and transmitter to supply the necessary energizing power to the system and to provide needed interfacing between the receiver and the two-wire link and transmitter.
2. Description of the Prior Art
It is well known in connection with the practice of current telemetering, particularly in the field of industrial process control, to provide a process condition transmitter, including a process condition value sensor, at the remote location at which the sensed condition exists, and to utilize a two-wire link to connect such a remotely located transmitter to a receiver which is located in a central control area. It is also well known to construct and energize such apparatus so that a current flows around the series path or loop including the transmitter, the two-wire link, and the receiver, and so that the transmitter causes the value of that current to be a desired function of the value of the sensed condition. The receiver in such known systems is constructed to translate the value of the loop current into an indication, record, and/or control function which represents the sensed value of the condition. It is further well known to utilize a power supply or current source at the receiver location to produce the loop current, and to utilize this current, by way of the two-wire link, to provide at the transmitter all of the power required to energize the latter. In so doing, the power supply at the receiver delivers a voltage to the two-wire link which produces the transmitter supply voltage at the transmitter. An example of such a current telemetering system, wherein the transmitter receives its supply voltage over the two-wire link, is the system disclosed in the Hurd U.S Pat. No. Re.27,596.
It has been found in practice to be desirable to include intrinsic safety Zener barriers in systems of the type described above in order to prevent the production of unsafe conditions in the vicinity of the portion of the current loop which includes the two-wire link and the transmitter. Such a Zener barrier usually consists of first and second stages, each of which includes a Zener diode. The known operating theory of such barriers requires that the Zener voltage rating of the second stage diode of a barrier be less than the Zener voltage rating of the first or input stage diode of that barrier.
In a system of the type just described, it has also been found to be desirable to provide interface apparatus between the power supply, the receiver, and the barrier/ two-wire link/transmitter portion of the system. An example of such interface apparatus, used in a current telemetering system including a Zener barrier as described above, is the apparatus which is disclosed in the Saul et al. U.S. Pat. No. 4,001,703, wherein the desired interfacing is accomplished by the use of a so-called current mirror circuit. As explained in that patent, such interface apparatus provides desired flexibility in regard to the supplying of the power to the system, in regard to grounding procedures for the system components, and in regard to the number of barriers required.
When interface apparatus of the above-noted type is employed in a system as just described, the voltage applied across the two-wire link by the interface apparatus is the source of the supply voltage at the transmitter. This applied voltage is by no means constant, and has been found in practice to exhibit excursions which cause the conduction or tripping of one or more of the barrier Zener diodes. Such diode tripping is undesirable, since it interrupts the system operation.
It has, therefore, been suggested to employ interface circuitry which holds the voltage at the barrier input constant at a value which prevents such diode tripping. This procedure has the practical disadvantage of unduly limiting the supply voltage available at the transmitter. This is so because the lower Zener voltage rating of the second stage barrier diode requires that the constant voltage at the barrier input, and hence the transmitter supply voltage, be kept lower than they would have to be if the point of controlled voltage were elsewhere, such as at the second stage barrier diode. This restricting of the transmitter voltage is undesirable, since it is desirable to have the transmitter voltage as high as the parameters of the system will permit.
It has been suggested previously to use a voltage regulated power supply for the entire telemetering system. This has the disadvantage of not necessarily preventing tripping of the barrier diodes.